1. Field of the Invention
This invention relates to a method for the preparation of hydrophobic reinforcing silica fillers for silicone rubber and to the fillers produced in accordance with said method.
Description of the Prior Art
Silicone rubbers formed from the vulcanization of polydiorganosiloxane gums alone generally have low ultimate elongation and tensile strength values. One means for improving the physical properties of silicone rubbers involves incorporation of a reinforcing silica filler into the gum prior to vulcanization. However, reinforcing silica fillers have a tendency to interact with the polydiorganosiloxane gum and this results in a phenomenon known as "crepe hardening." A great deal of effort has been made in the past to treat the surface of reinforcing silica fillers with organosilanes or organosiloxanes to render the surface of the silica hydrophobic. This surface treatment reduces or eliminates the tendency of the compositions to crepe harden while improving the physical properties of the vulcanized silicone rubber.
Brown, in U.S. Pat. No. 3,024,126, teaches a method for rendering a pre-formed reinforcing silica filler hydrophobic by treating it in an organic solvent with an organosilicon compound, such as an organosilane or low molecular weight organosiloxane containing 0.1 to 2 total hydroxyl and/or alkoxy radicals per silicon atom, and a small amount of a certain amine, quarternary ammonium or organometallic compound. Lewis, in U.S. Pat. No. 3,979,546, teaches a method for rendering the surfaces of reinforcing agents such as siliceous fillers hydrophobic through the use of alpha-alkoxy-omega-siloxanols prepared by the reaction of cyclic siloxanes with alcohols under mild conditions. The fillers taught are pre-formed solids. However, these methods require that the reinforcing silica filler be manufactured first and then surface-treated in the form of a powder or gel.
Tyler, in U.S. Pat. No. 3,015,645 teaches the production of hydrophobic silica powders by reacting an organosilicon compound such as dimethyldichlorosilane or trimethylmethoxysilane with an organogel in the presence of an acidic catalyst and then removing the volatile materials. This requires the preparation of a hydrogel which is converted to an organogel by replacing the water in the hydrogel with an organic solvent. Lentz, in U.S. Pat. No. 3,122,520, teaches a procedure wherein an acidic silica hydrosol is first heated to develop a reinforcing silica structure and then mixed with an organosilicon compound, such as the ones taught in Tyler above, an acidic catalyst and a water-immiscible organic solvent to produce a hydrophobic silica filler.
In U.K. Patent Application No. GB 2,001,303A, a hydrophobized precipitated silica is prepared by precipitating waterglass in a weakly alkaline aqueous medium. After precipitation of the silica, an alkaline pH is maintained while an organosilicon hydrophobing compound of the general formula (Y'.sub.3 Si).sub.a Z' is added to the medium containing the silica. Y' is a monofunctional hydrocarbon radical, Z' is --OH, --OY, or --NYX, among others, and X is hydrogen or Y'. The three preceding processes have the disadvantage of having to deal with the disposal of salt-containing solutions formed when the silica is precipitated and require the formation of a structured silica product prior to the hydrophobization step.
One attempt to eliminate the separate hydrophobization step is found in British Pat. No. 1,062,599 wherein a hydrophobic silica is prepared by precipitating silica from an aqueous waterglass solution in the presence of an organohalosilane. The precipitation is conducted under acidic conditions. This process still produces a water solution containing salts as a by-product.
In U.S. Pat. No. 3,850,971 entitled "Porous Silicic Acid and Its Production", Termin et al. describe a process for producing porous silicic acid which is free of ions or salts through the hydrolysis of silicic acid esters in the presence of 70 to 120% of the stoichiometrically required amount of water with moderate stirring using a particular type of stirring apparatus as detailed in the specification. Up to 100% alcohol by volume, relative to the amount of silicic ester or polyalkyl silicate, can be added to facilitate homogeneous mixing of the silicic acid ester, preferably tetramethyl silicate, and water. A preferred method involves the use of 0.1 to 2 mole percent, relative to the amount of silicic ester or polyalkyl silicate, of a hydrolysis activator such as acidic or basic substances. Example 19 employs approximately twice the suggested maximum amount of hydrolysis activator (ammonia), but no reason is given for this deviation. Hydrolysis activators such as compounds containing silicon or transition metals are preferred because they act as activators, enter into the product without causing undesirable contamination and can influence the silicic acid product relative to wettability. Silyl amines and silyl azanes are suggested in U.S. Pat. No. 3,850,971 as being some of the suitable basic substances and trimethoxy-gamma-amino-propyl silane is suggested as being useful to influence the water repellency of the product. Many possible uses for the silicic acid gels formed are mentioned such as fillers for paper, rubber, plastics, filter aids and in chromatography. U.S. Pat. No. 4,006,175 is a division of the above patent to Termin et al. and involves a process for producing porous silicic acids containing oxides of transition metals.
However, the Termin et al. patents only teach the use of silicon compounds which are either basic or which liberate basic substances as hydrolysis activators and does not suggest the use of a sufficient amount of such compounds to render the silicic acid product or silica filler sufficiently hydrophobic to be useful as a hydrophobic reinforcing silica filler for silicone rubber. The Termin et al. patents do not suggest that basic substances such as ammonia can be used in combination with a sufficient amount of a silicon compound which would not be considered either to be a basic substance or to be a substance which would liberate a basic substance such as dimethyldimethoxysilane to produce a hydrophobic reinforcing silica filler for silicone rubber by the process hereinafter described. Furthermore, the Termin et al. process appears to require a particular type of stirring apparatus. Furthermore, the patent does not suggest that amounts of water in excess of 120% of the stoichiometric amount are desirable in producing the fillers described therein and furthermore does not suggest that amounts of water greater than 120% can be used to produce hydrophobic reinforcing silica fillers for silicone rubber.
In U.S. Pat. No. 4,017,528 entitled "Preparation of Organically Modified Silicon Dioxides," Unger et al. teach the preparation of modified, porous silicon dioxides by the hydrolytic polycondensation of tetraalkoxysilanes or polyalkoxysiloxanes in the heterogeneous phase in the presence of an organoalkoxysilane. One embodiment of the process (Method 2) involves mixing a polyalkoxysiloxane (or alkyl polysilicate) having an average molecular weight of 750 to 2800 with an organoalkoxysilane (preferably A.sub.n Si(OB).sub.4-n where A is an alkyl, aryl or aralkyl group, B is an alkyl group of 1-4 carbon atoms and n is an integer 1, 2 or 3, preferably 1), introducing this mixture into a mixture of a large excess of water and a water-miscible solvent, such as methanol or ethanol, to form a heterogeneous mixture, dispersing the heterogeneous mixture by agitation or shaking (the Examples describe the process as one whereby the solutions are "emulsified") and then adding a basic catalyst to the dispersion. Upon addition of the catalyst, hydrolytic polycondensation is said to occur and allegedly produces spherical particles of organosilicon dioxide gel which are useful as chromatographic adsorbents. The patent teaches that hydrophobicity of the organosilicon dioxide gel can be altered by the choice of organic group present in the organoalkoxysilane used in the process. The process described in the Unger et al. patent is also noted in an article by Unger et al. entitled "Recent Developments in the Evaluation of Chemically Bonded Silica Packings for Liquid Chromatography," Journal of Chromatography, Vol. 125, pp. 115-127, Elsevier Scientific Publishing Company, Amsterdam (1976). This article deals with the use of surface modified silicon dioxides as chromatographic adsorbents and one method for preparing such modified silicon dioxide appears to be the method described in the Unger et al. patent.
However, the Unger et al. patent is directed towards the preparation of organosilicon dioxide gels useful in chromatographic applications and does not suggest that the allegedly spherical products produced by the hydrolytic condensation in the heterogeneous phase will possess the degree of hydrophobicity and the type of structure required to enable the products to be useful as a hydrophobic reinforcing silica filler for silicone rubber. Furthermore, the Unger et al. patent teaches that the preferred organoalkoxysilanes are those where n has a value of 1, but for purposes of rendering silica fillers hydrophobic, n preferably has a value of 2 or 3.
From the foregoing discussion, it is apparent that there is a need for a simple procedure for preparing a silica filler possessing both the requisite amount of hydrophobicity and the necessary structure to be useful as a hydrophobic reinforcing silica filler for silicone rubber. Such a process should avoid the necessity for producing a silica filler with a structure suitable for use as a reinforcing filler in silicone rubber prior to the hydrophobization step. Such a process should also be one which does not produce by-products such as salts which would contaminate the silica filler and pose disposal problems. A novel process for obtaining such fillers will now be described.